This application claims the benefit of Korean Patent Application No. 1998-40213, filed on Sep. 28, 1998, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method for crystallizing a silicon film and a thin film transistor and fabricating method using the same and, more particularly, to a method for crystallizing a silicon film and a method for fabricating a thin film transistor using a laser annealing technique.
2. Discussion of the Related Art
When certain energy, such as laser energy, is applied to amorphous silicon, the amorphous silicon melts and as it cools or solidifies, silicon grains grow and crystallize. Polycrystalline silicon or single crystalline silicon is determined depending on the state of growth of the silicon grain. Single crystalline silicon is obtained in case each silicon grain grows in a single direction and polycrystalline silicon is obtained in case each silicon grain grows randomly at the same time.
For a thin film transistor whose active layer is formed by crystallizing an amorphous silicon film in order to enhance the characteristics of the thin film transistor, it is preferable to reduce the number of grain boundary, which inhibits migration of carriers, by increasing the size of the silicon grain.
FIGS. 1A to 1D are schematic diagrams for explaining a method for crystallizing an amorphous silicon film according to related art. Referring to FIG. 1A, a first insulation film 11 and an amorphous silicon film 12 are successively deposited on an insulating substrate 10. An oxide film which exhibits half-reflection is deposited on the amorphous silicon film 12 and then etched by lithography to form a half-reflection layer 13. Because the half-reflection layer 13 half-reflects an incident laser on the amorphous silicon film 12, the portion of the amorphous silicon film 12 which underlies the half-reflection layer 13 gets hot fast but cools slowly. Hereinafter, the portion of the amorphous silicon film 12 with the overlying half-reflection layer 13 is called a first silicon region 12-1 and the other portion of the amorphous silicon film 12 without the overlying half-reflection layer 13 is called a second silicon region 12-2.
Referring to FIG. 1B, a laser beam is irradiated to the entire surface of the substrate 10. Here, the energy of the laser beam is controlled to such a level that the first silicon region 12-1 is completely melted but the second silicon region 12-2 contains a predetermined number of unmelted silicon particles 14.
Referring to FIG. 1C, the irradiated amorphous silicon by laser beam is immediately cooled to have the silicon particles grow, thereby, to perform silicon crystallization.
During the crystallization, the silicon particles 14 remaining in the first silicon region 12-1 serve as seeds for growing the silicon grains and are grown. The silicon grains stop growing as they collide with one another. Crystallization occurs at many locations of the silicon film at the same time according to the locations of the growth seeds, so that the second silicon region 12-2 becomes a first polycrystalline silicon film 15 in which the silicon grains are randomly positioned. In contrast, the first silicon region 12-1 remains in a molten state due to the half-reflection layer 13 that retards cooling of the first silicon region 12-1.
Referring to FIG. 1D, an interface between the solid first polycrystalline silicon film 15 and the liquid first silicon region 12-1 becomes a crystal seed that provides a lateral growth of the grains developing from the boundary of the first polycrystalline silicon film 25. As a result, a silicon grain boundary is laterally positioned. Here, the silicon grains grown on both boundary sides meet together and stop growing at the center of the first silicon region 12-1. Thus, the first silicon region becomes a second polycrystalline silicon film 16 in which the grains are much larger in size than those of the first polycrystalline silicon film 15.
However, the silicon grains of the second polycrystalline silicon film 16 obtained in the related art have a size not exceeding a maximum of 1 xcexcm under the processing atmosphere, at a room temperature or less than 400xc2x0 C.
When the second polycrystalline silicon film having silicon grains larger than 1 xcexcm in size is formed, as shown in FIG. 2, fine silicon grains are formed at many positions at the center of the second silicon region. Correspondingly, the polycrystalline silicon film crystallized by the related art technique is hard to use for an active layer of thin film transistor, considering that ordinary thin film transistor has a channel length of about 10 xcexcm.
Accordingly, it is an object of the present invention to provide a silicon film crystallization method and a method for fabricating a thin film transistor using the same.
It is another object of the present invention to provide a silicon film crystallization method by which silicon grains can be dramatically increased in size.
Further, it is another object of the present invention to provide a method for fabricating a thin film transistor with an enhanced characteristic of the device, in which a silicon with silicon grains dramatically increased in size is used as an active layer of the thin film transistor.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve the first object of the present invention, there is provided a method for crystallizing an amorphous silicon film including the steps of: preparing a substrate having the amorphous silicon film, the amorphous silicon film being formed on an intermediate layer which defines an inner space between the substrate and the intermediate layer; and applying an energy to the amorphous silicon film in order to crystallize the amorphous silicon film. The step of preparing the substrate includes the steps of: forming a material layer for forming the space on an insulating substrate, forming the intermediate layer to cover the material layer, forming the amorphous silicon film on the intermediate layer, selectively removing the amorphous silicon film and the intermediate layer to expose a part of the material layer for forming space, and removing the material layer for forming space; or forming a material layer for forming the space on an insulating substrate, forming the intermediate layer to cover the material layer, selectively removing the intermediate layer to expose a part of the material layer, removing the material layer, and forming the amorphous silicon film on the intermediate layer.
In another aspect of the present invention, there is provided a method for fabricating a thin film transistor including the steps of: preparing a substrate having the amorphous silicon film formed on an intermediate layer provided with an inner space at a predefined position thereof; applying a laser energy to the amorphous silicon film to crystallize the amorphous silicon film and form a polycrystalline silicon film; photo-etching the polycrystalline silicon film to form an active layer; forming a gate insulation film and a gate electrode on the active layer; forming a passivation layer covering the exposed entire surface of the substrate including the gate electrode; photo-etching the passivation layer to expose a part of the active layer; and forming source and drain electrodes connected to the exposed active layer.
In another aspect of the present invention, there is provided a thin film transistor including: an insulating substrate; an intermediate layer formed on the insulating substrate and having an inner space at a predefined position thereof; an active layer formed on the intermediate layer over the space; a gate insulation film and a gate electrode formed on the active layer; and source and drain electrodes connected to the active layer.
In another aspect of the present invention, a method of crystallizing an amorphous silicon layer comprises: forming an intermediate layer on a transparent substrate; forming an amorphous silicon layer on the intermediate layer; forming a space within the intermediate layer; and applying energy to the amorphous silicon layer for crystallizing the amorphous silicon layer.
In a further aspect of the present invention, a method of crystallizing an amorphous silicon layer comprises forming a material having a first heat conductivity on a transparent substrate; forming an intermediate layer having a second heat conductivity on the material and the transparent substrate, the first heat conductivity being less than the second heat conductivity; forming an amorphous silicon layer on the intermediate layer; applying energy to the amorphous silicon layer for crystallizing the amorphous silicon layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.